The Bittersweet Promise of Glycobiology

The bittersweet promise of glycobiology

Carbohydrates play a wide range of important roles in the body and, despite the challenges posed by glycobiology, are now tempting targets for drug developers.

Alan Dove

Although several genomes have been sequenced and many protein structures Lumen solved, there has been relatively little attention ER Golgi paid, to date, to the various ways in which proteins are “tweaked” through the attachment of sugars. However, the process of glycosylation is far from a decorative function. Carbohydrates help determine the three- dimensional structures of proteins, which are inherently linked to their function and their efficacy as therapeutics. Moreover, in contrast Synthesis of Glycan Trimming Further Terminal lipid-linked transfer and trimming glycosylation to some of the other chemical tags employed precursor processing by cells (e.g., phosphates and lipids), carbohy- = glucose = fucose drates exhibit a mind-boggling diversity of = mannose = galactose structures, can confer cell-type specificity, and Cytosol = N-acetylglucose = sialic acid are crucial components of cell-to-cell signal- ing. At the same time, carbohydrates make © Bob Crimi problematic drug targets; they are the most Figure 1. A simplified diagram showing the synthesis of an N-linked glycan. First, sugars are linked difficult biological molecules to analyze and onto a lipid precursor (in the cytosol), which is then flipped over into the lumen of the endoplasmic synthesize, and are rapidly broken down in the reticulum (ER) and the core oligosaccharide finished. The glycan is then transferred to the nascent, growing polypeptide. Sugars are trimmed off, and the polypeptide is then folded (grey oval) before bloodstream. Despite these challenges, recent being moved to the Golgi complex. The glycoprotein goes through a series of further modifications, technological advances have enabled several ending with the capping of the oligosaccharide branches with sialic acid and fucose. Source: biotechnology companies to pursue new car- Helenius, A. & Aebi, M. Science, 291, 2364Ð2369, 2001. © http://biotech.nature.com Group 2001 Nature Publishing bohydrate-based products, sweetening the outlook for glycobiology. the structures back so that new sugars can can have disastrous consequences. For Carbo loaded be added to a different part of the growing example, in Gaucher’s and Fabry’s diseases, In contrast to DNA, RNA, and proteins, car- carbohydrate structure. The back-and- inherited mutations in carbohydrate-pro- bohydrates can form branching structures, forth process of glycosylation continues cessing enzymes cause glycolipids to accu- and for this reason a relatively simple set of into the Golgi apparatus, which sorts the mulate, leading to damage to the kidneys, sugars can form a huge number of complex new proteins, distributing them to their heart, and brain. The inability to form cer- structures. For example, in theory, the nine final destinations in the cell. In human tain glycoproteins results in another set of common monosaccharides found in cells, sialic acid is usually added to the tips inherited diseases called congenital disor- humans could be assembled into more than of the carbohydrate branches, serving as a ders of glycosylation, which can result in 15 million possible tetrasaccharides, all of final cap important for a variety of glyco- symptoms ranging from chronic diarrhea which would be considered relatively simple protein functions (see Fig. 1). to life-threatening neurological problems. glycans. Different cell types express different Carbohydrates can significantly affect In addition to relatively rare genetic dis- complements of glycosylating enzymes, and protein structure and therefore influence eases, sugars play crucial roles in some have different sets of proteins that can be protein function. Carbohydrates are also leading health problems. Injured tissues glycosylated. Glycosylation is therefore a used as tags to sort proteins in the Golgi stimulate endothelial cells to express flexible evolutionary tool serving a wide apparatus, targeting them to specific com- selectins—glycoproteins that tell leuko- range of purposes in complex organisms. partments within the cell or directing them cytes in the bloodstream to infiltrate dam- The process of glycosylation begins to the cell surface. Once on the cell mem- aged tissue and mount an inflammatory when the protein is targeted to the glycosy- brane, glycoproteins (and glycolipids) response. However, this inflammation lation pathway during translation of mes- interact with receptors on other cells to process can occasionally get out of control. senger RNA (mRNA) into protein. The communicate a wide range of messages. In surviving stroke and heart attack ribosome is attached to the endoplasmic For example, cells of the immune system patients, for example, blood flow is briefly reticulum (ER), and the nascent protein use the glycans on the cells that they cut off from a tissue, then restored. As the fed into the lumen of the ER as translation encounter to identify everything from bac- blood starts to flow again, selectins in the proceeds. In the ER, one set of glycosyla- terial invaders to fellow leukocytes. affected tissue aggressively recruit leuko- tion enzymes attaches sugars to specific cytes from the bloodstream, leading to an portions of the protein. Other glycosyla- Sweet but sickly intense inflammatory response. The result- tion enzymes then either add more sugars Not surprisingly, given the importance of ing tissue damage, so-called reperfusion to these core structures, or partially trim carbohydrates, defects in their metabolism injury, is typically more severe than that

sustained by the initial interruption of of possible carbohydrate structures, and The mixture of carbohydrates can then be blood flow. If the glycan-mediated signal the cumbersome technology available, gly- subjected to high-performance liquid chro- from the selectins could be modulated, the cobiology has been a tough sell for biotech- matography (HPLC) to separate out individ- damage might be mitigated. nology and pharmaceutical companies, ual carbohydrates. Finally, individual carbo- Carbohydrates are also important for made even tougher by a series of high-pro- hydrates can be studied in detail by chemical tumor development because cancer cells file clinical trial failures during the mid- digestion and mass spectrometry, a tech- alter their surface glycoprotein expression 1990s (see below). nique that can give precise structural infor- to evade the immune system. By cloaking mation about relatively small molecules themselves with the right assortment of such as monosaccharides. glycoproteins, the tumor cells can invade I would say that glycoproteomics Besides the complex multistep analysis other tissues without being identified as required for each sugar, glycobiologists aliens. is at least an order of magnitude must also contend with the heterogeneity Sugar structures are also targets for more difficult than proteomics of sugar structures within a sample. For pathogens seeking entry into the cells of example, Pauline Rudd, a scientist at the their host. Because carbohydrates are both Glycobiology Institute at the University of ubiquitous and diverse, viruses and other Oxford (UK), points out that the human intracellular parasites can use glycoprotein Unlike DNA and proteins, no technology prion protein can carry one of 52 different receptors to target any host cell, or just one has been developed to allow carbohydrates sugar structures at one site on the protein: cell type in particular. to be “sequenced” using straightforward “That, of course, makes it very difficult to techniques. Instead, sugar structures must be analyze, because if you’ve got ‘x’ amount No simple solution solved by a combination of chromatography [of sample], you’ve got ‘x’ divided by 52 of For many years, the potential of carbohy- and mass spectrometry, procedures that have any one carbohydrate.” Obtaining suffi- drate-based therapies has been yoked to traditionally required considerable technical cient quantities of a given glycan for analy- seemingly intractable technical problems. skill. “I would say that glycoproteomics is at sis is often a major problem, especially in Without high-throughput methods for least an order of magnitude more difficult studying glycoproteins present in low con- analyzing and synthesizing carbohydrates, than proteomics,” says Anne Dell, a centrations in cells. “We can look at the detailed knowledge of the structure and researcher in the department of biochem- things that are there in very high abun- function of each carbohydrate moiety has istry at the Imperial College of Science, dance, but not at the more interesting been the hard-won products of labor- Technology, and Medicine (London, UK). In things are generally there in low abun- intensive experiments. Indeed, even the a typical experiment, a glycoprotein is isolat- dance,”says Rudd. nomenclature of carbohydrates inhibits ed from a chromatography gel, and the car- Once a carbohydrate structure is solved, easy understanding (see “By any other bohydrate groups are then released from the duplicating the structure artificially can also name”). Given the extraordinary diversity protein by chemical or enzymatic treatment. be a daunting task. Cells that are capable of © http://biotech.nature.com Group 2001 Nature Publishing

By any other name

Among the many difficulties that have faced glycobiologists, one of the most obvious may also be the easiest to fix. Molecular biologists use simple alphabetic codes to describe the nucleic acid or protein sequences they study, but glycobiologists have been stuck using the complete, clumsy chemical names of their complex molecules. In an effort to improve clarity, Glycominds (Maccabim, Israel) has developed the Glycominds Linear Code, a nomenclature that allows any sugar to be described with a code of letters and punctuation that accurately represents its building blocks and branching structure. “The Glycominds Linear Code builds a unique syntax that can describe any glycan in just one way,” says Avinoam Dukler, CEO of Glycominds. The Glycominds Linear Code is a proprietary system, but Dukler says that it is licensed at no charge to any researcher requesting it. Another unique advantage is that code is computer-readable, which no other glycan- naming system can claim. “Now the world of computation and bioinformatics can begin [for glycobiology],” says Dukler. AD

glycosylating proteins generate a diverse major drug delivery problem for ordinary array of carbohydrate structures, and it is protein therapeutics, most of which must difficult to control the production of a spe- be correctly glycosylated to work. cific glycoprotein in vivo. Until recently, Glycosylation also affects protein break- organic chemistry was the most common down; for example, proteins lacking termi- approach for carbohydrate synthesis in the nal sialic acid residues on their sugar lab, but the complexity of biologically rele- groups are often targeted by the immune vant glycans pushes synthetic chemistry to system and rapidly degraded (see “Sugar in its limits. In addition, the synthesis of each bulk”). structure had to be designed from scratch, Glycobiology is “a bit like the proverbial © GlycoDesign making this approach inappropriate for onion. You get through one layer and you A model of a carbohydrate that is a substrate for high-throughput drug design. discover that there are whole new layers of a glycosylating enzyme, GnT-V. Bubbles depict The struggle does not abate once a poten- complexity to be addressed,” says binding site. tially useful carbohydrate is synthesized. GlycoDesign’s Cumming. One layer of the “Using carbohydrates themselves as thera- onion that left a bitter taste in the mouths bly have to prove itself with a significant peutics … there are some fairly unique of pharmaceutical companies and investors clinical success before large companies and issues,” says Dale Cumming, vice president were early failures. In the early 1990s, investors will be willing to return to the for discovery research at GlycoDesign biotechnology startup Cytel (San Diego, field. (Toronto, ON, Canada). The digestive sys- CA) set out to develop carbohydrate tem excels at breaking down most naturally inhibitors of selectins, the glycoproteins Glycan do it occurring carbohydrates, and such drugs that promote inflammation. The company’s Glycobiologists seem to have risen to the will likely have to be injected directly into the lead drug—Sialexin—failed in clinical tri- challenge, making several crucial techno- bloodstream. Glycosidases in the blood can als. A double blow came when a second gly- logical advances in the past few years and reduce a carbohydrate-based drug’s half-life cobiology startup, Glycomed, failed in its overcoming at least some of the obstacles to just a few minutes, depending on its struc- efforts to develop a modified form of facing early pioneers. The major technolo- ture. Carbohydrate-based drugs such as heparin. The two companies were subse- gy for analyzing sugar structures—mass heparin—the world’s best-selling prescrip- quently bought out by competitors, but spectrometry—has improved in both sen- tion drug—and the Haemophilus influenzae their story cast a long shadow over the field sitivity and accuracy, and has become tech- type B (Hib) vaccine have avoided this phar- of glycobiology. “They spent a lot of money nically easier, making carbohydrate analy- macokinetic pitfall because their structures and a lot of years in clinical trials that sim- sis accessible to more researchers outside of are not readily recognized by the body’s nor- ply demonstrated no efficacy whatsoever,” the field of glycobiology. In Oxford, Rudd mal complement of glycosidases. However, says Stephen Roth, chairman and chief and her colleagues regularly teach their evidence is mounting that other carbohy- executive officer (CEO) of Neose analytical glycan extraction and mass spec- © http://biotech.nature.com Group 2001 Nature Publishing drate-based therapeutics will not fare as well. (Horsham, PA), which purchased Cytel. As trometry techniques to visiting scientists, Indeed, carbohydrates also present a a result, says Roth, glycobiology will proba- who can then incorporate glycan analysis into their regular work. While conceding that it is still far from a turnkey technology such as DNA sequencing, Rudd argues that Sugar in bulk this type of analysis will ultimately become Biotechnology companies working on glycans are primarily interested in developing new part of the molecular biologist’s toolkit. drugs (see main text), but at least one company is pursuing broader applications. Neose Another analytical technique for carbo- (Horsham, PA) has established partnerships with at least two dozen pharmaceutical and hydrates may be widely available sooner. biotechnology companies to improve the glycosylation of medically useful glycoproteins.The Glycominds (Maccabim, Israel) has devel- company is also developing strategies to drastically slash the cost of protein production, for oped a chip-based assay for glycan binding, both pharmaceutical and industrial uses. an important first step in determining the Neose’s major technology, Glycoadvance, uses enzymes to add sugars to proteins in function of glycans. The chip allows the vitro, significantly improving the yield of usable protein from fermentations. There has been researcher to screen the carbohydrate- no shortage of interest in the service: “Most large pharmaceutical companies and big biotech binding pattern of a protein against a few companies have products that are sort of stalled somewhere in advanced clinical trials hundred unique carbohydrate structures because of protein production problems. Those production problems always come back to simultaneously. The pattern of colored less-than-optimal glycosylation of their product by the Chinese hamster ovary (CHO) cells,” spots on the chip indicates which sugar says Stephen Roth, chairman and CEO of Neose. structures bind to the protein.“We’re using Biotechnology companies have long used fungal systems like yeast and Aspergillus to these carbohydrates as a tool to identify manufacture huge quantities of proteins cheaply. However, fungi glycosylate proteins with a novel proteins that have carbohydrate- lot of mannose, which makes the protein therapeutically useless: The human immune binding domains. For drug discovery, we’re system identifies this glycosylation pattern as that of a fungal or bacterial invader, rapidly screening low-molecular-weight com- removing the proteins from the circulation. By adding humanlike sugar groups, such as sialic pounds that might inhibit these interac- acid, to proteins produced in yeast, Roth hopes to eliminate the troublesome and expensive tions,” says Avinoam Dukler, CEO of CHO cells from the production process. Glycominds. The chips might also form the If that strategy works, the impact on the cost of protein drug production could be basis of a diagnostic tool to screen for anti- substantial. “The cost of producing a typical pharmaceutical glycoprotein [in CHO cells] will bodies against a particular glycan. be between $1 million and $3 million a kilogram.Your typical industrial glycoprotein made in In addition to commercial applications Aspergillus will be around $200Ð$300 a kilogram,” says Roth. AD for its chip, Glycominds is hoping to aid academic and government scientists profil-

Plante, are cofounders of Advanced Carbohydrate Technologies (Cambridge, MA), which is commercializing the technology for drug development and research. In enzymatic synthesis, Neose has enjoyed considerable success (see “Sugar in bulk”), but Roth concedes that “the place where we are relatively helpless is the synthesis of complex carbohydrates that do not exist in nature, and for which there are no [glycosylating] enzymes.” That limitation may soon be lifted, at least partially. By rationally mod- © GlycoMinds ifying one of the key glycosylation enzymes The GlycoChip is the first microarray for complex carbohydrates. Synthesis of the glycans on the chip in Salmonella bacteria, researchers at the is done in situ . Memorial Sloan–Kettering Cancer Center (New York, NY) and Brookhaven National Laboratory (Brookhaven, NY) created an ing carbohydrate expression and binding reactions are carried out in a single test enzyme that uses sugars that are not natural patterns. One such effort, the Consortium tube to produce the final carbohydrate substrates for a wild-type glycosylating 2 for Functional Glycomics (http://function- product; in solid-phase synthesis, the ini- enzyme . Non-natural sugar structures are of alglycomics.mit.edu), recently applied for tial sugar groups are anchored to beads or particular interest to drug developers a National Institutes for Health grant to surfaces, allowing them to be isolated and because they may resist degradation in the finance its multi-institution glycobiology moved easily at each step of the process; bloodstream. project. Stressing that the project has not enzymatic processes use glycosylating Biological carbohydrate synthesis strate- yet been funded, James Paulson, professor enzymes, derived from cells, to carry out gies have focused primarily on producing of molecular biology at The Scripps the reactions; finally, several groups have better glycoproteins, rather than generat- Research Institute (La Jolla, CA) and one developed novel biological systems, in ing pure carbohydrates. Genentech (S. San of the initiators of the consortium, says which carbohydrates are produced in Francisco, CA), for example, has developed that it is “interested in defining the para- genetically modified animal, yeast, or bac- genetically modified Chinese hamster digms through which carbohydrate bind- terial cells. Each method has advantages ovary (CHO) cells that attach more sialic ing modulates [protein] function and in and disadvantages, but all of them have acid to their glycoproteins than ordinary particular modulates communication been improved in recent years. CHO cells, improving the pharmacokinetic between cells.” A major advance in solid-phase synthe- profiles of the manufactured proteins3. © http://biotech.nature.com Group 2001 Nature Publishing sis was the development of an automated Advances such as these will undoubtedly Sugars by design system that may eventually become as accelerate the pace of glycobiology While new techniques are beginning to widely used as oligopeptide and oligonu- research, but most experts agree that suc- yield more information about glycan struc- cleotide synthesizers1. “What we have cessful labs will eventually employ a mixture and function, improved means of syn- developed is a very general procedure in ture of techniques to synthesize glycans thesizing carbohydrates for further study the lab...based on an application which and glycoproteins. and drug development are also emerging. builds on monosaccharide molecules,” says The new synthetic approaches fall into four Peter Seeberger, professor in the depart- Developing sugar pills major categories: One-pot synthesis is a ment of chemistry at the Massachusetts Even though most large pharmaceutical direct descendant of traditional organic Institute of Technology (Cambridge, MA). companies are still wary of glycobiology, it chemistry techniques, in which successive Seeberger and one of his students, Obadiah seems clear that carbohydrates will play an

Table 1. Companies focusing on carbohydrates Company (location) Focus

Abaron Sciences (La Jolla, CA) Targeting glycosylation enzymes for a variety of novel therapies. Biomira (Edmonton, AB, Canada) Vaccines like Theratope, to halt metastatic cancers. GlycoDesign (Toronto, ON, Canada) Carbohydrate-processing inhibitors (CPIs) for the treatment of cancer, infection, inflammation, and cardiovascular disease: GD39 in phase 2 for various metastatic cancers. Glycominds (Maccabim, Israel) High-throughput study of complex carbohydrates. Products include GlycoChip and GlycoSd with Glycominds database. GlycoTech (Rockville, MD) Using non-natural molecules to mimic carbohydrates. Neose Technologies (Horsham, PA) Enzyme-based protein glycosylation and bulk sugar synthesis. In conjunction with Bristol-Myers Squibb, have a melanoma cancer vaccine in phase 3. Neuronyx Biotech (Malvern, PA) Glycolipid research. Novazyme (Oklahoma City, OK) Glycosylation enzymes and lysosomal storage diseases. Progenics (Tarrytown, NY) Ganglioside-based vaccine for the treatment of melanoma in phase 3. SafeScience (Boston, MA) Carbohydrate-based cancer treatments. GBC-590 in phase 2 for treatment of pancreatic and colorectal cancers. Synsorb Biotech (Calgary, AB, Canada) Oligosaccharides to bind toxins in the gut; Synsorb Cd in phase 3 for treatment of Clostridium difficile infection. Synthon Chiragenics (Monmouth Junction, NJ) Carbohydrate-based chiral chemistry technology.

increasingly important role in future drug inhibitor of cellular glycosylation enzymes in check or possibly even eliminated. development, and a number of biotech- leads to the mis-glycosylation and subse- In addition to helping cancer evade the nology companies have begun to dabble in quent misfolding of the matrix protein of immune system, glycans help orchestrate it (see Table 1). In addition to heparin and hepatitis B virus. Host cell proteins are also normal and pathological immune respons- the Hib vaccine, other successful mis-glycosylated when the drug is given, es, a fact that is also drawing the attention carbohydrate-based drugs include the but the cell can identify and eliminate the of researchers. Many autoimmune diseases influenza antivirals Tamiflu and Relenza defective proteins, whereas the virus can- are driven by a process called the Th1 and the antibiotics erythromycin and van- not. The resulting defective viral particles response, in which the immune system ini- comycin. are unable to initiate another round of tiates a chain reaction of inflammatory sig- Biotechnology companies are also start- replication, and glycosylation inhibitors nals. Donald Harn, a researcher at Harvard ing to pay more attention to glycosylation, appear to be promising antivirals4.The University (Cambridge, MA), is hoping to which is now being recognized as a major same group is also pursuing a similar use glycoconjugates to initiate compen- problem in protein production. Amgen approach for HIV treatments, although satory anti-inflammatory Th2 responses: (Thousand Oaks, CA), for example, dis- neither therapy has reached clinical trials “We’ve now done a large number of studies cards about 80% of the erythropoietin it yet. One advantage of targeting the glyco- in three different animal models in which produces because the protein is not cor- sylation enzymes, rather than the carbohy- ... the glycoconjugates appeared to be ther- rectly glycosylated. Like many other thera- drates themselves, is that small-molecule apeutic.” Harn, whose laboratory is also peutically important proteins, erythropoi- enzyme inhibitors can be developed and collaborating with Neose to develop adju- etin will not function correctly if it lacks delivered in the same manner as traditional vants for improved vaccines, hopes to use the right sugar groups. Although the pharmaceuticals, avoiding the difficulties similar carbohydrate-based drugs to mod- importance of glycosylation has been of delivering carbohydrate-based drugs. ulate a wide range of immune responses. understood for decades, companies have Even selectins, which led to the downfall tolerated low yields of recombinant pro- of Cytel, are getting a second chance. After teins in the past. However, with more pro- For biotechnology companies purchasing Cytel, Neose retained some of teins in their pipelines and fermentation the company’s research staff and continued facilities strained to their limits, companies glycobiology is an area really working on the selectin problem. Neose’s are looking for ways to improve efficiency. ripe for potential therapeutic Roth explains that when Cytel entered the Carbohydrates are also increasingly field the structure of the carbohydrate that being targeted as potential new medicines. intervention interacts with P-selectin had not been com- Carbohydrate-based antibiotics are well pletely solved: “Cytel chose a structure that established in medicine, but recently looked like the right structure at that time, researchers have taken a new strategy. Targeting cancers but ended up not being the correct struc- Pathogenic bacteria often use glycoprotein Another effort to target glycosylation ture.” In the ensuing years, says Roth, © http://biotech.nature.com Group 2001 Nature Publishing receptors on the surfaces of host cells to enzymes is taking place at GlycoDesign, Neose has discovered that “the real struc- colonize a tissue. Eric Vimr, a researcher at which is carrying out clinical trials on its ture is at least a thousand times more the University of Illinois (Urbana- lead anticancer compound, GD0039. active,”and the company is now developing Champagne, IL), says that new antibiotics GD0039 blocks a critical carbohydrate- it as an anti-inflammatory drug. might act to prevent this attachment: processing enzyme, and inhibits the ability While more companies are beginning to “[One could] design some oligosaccharide of cancer cells to evade the immune sys- target glycosylation and carbohydrates for that is identical or mimics that docking tem. The result seems to be a cellular stale- drug development, experts in the field agree molecule on the host cell ... to thwart the mate, in which a tumor is not able to grow that sugars will continue to present a bigger ability of the microorganism to colonize.” but is also not destroyed entirely. challenge than DNA or proteins. Carbohydrates are well tolerated by the Cumming explains that “we don’t know the “Glycobiology started in a less mature state human digestive system, and so carbohy- precise mode of action or even if there is a in the early 1990s than did proteins or nucle- drate-based medicines could have fewer singular mode of action [for the drug].” ic acids,” says Cumming, adding that “it’s side effects than traditional antibiotics for Although the importance of glycosylation clear we understand a lot more now than we treating gastrointestinal infections. for normal biology would suggest that did 15 years ago, but it’s clear there’s a lot Unfortunately, these so-called gly- inhibitors of glycosylation might have sig- more to understand. For biotechnology comimetic antibiotics have had little suc- nificant and undesirable side effects, companies, glycobiology is an area really ripe cess so far, probably because the individual GlycoDesign has found that, so far, the for potential therapeutic intervention.” sugar molecules can only bind individual drugs are well tolerated in experimental receptors on the bacteria, leaving adjacent, animals and in humans. 1. Plante, O.J., Palmacci, E.R. & Seeberger, P.H. unoccupied receptors free to attach to host Another class of carbohydrate-based Automated solid-phase synthesis of oligosaccha- cells. Vimr hopes to address this problem cancer treatments aims to alert the rides. Science 291, 1523Ð1527 (2001). by genetically modifying benign bacteria to immune system to the tumor’s glycan dis- 2. Barton, W.A. et al. Structure, mechanism and engineering of a nucleotidylyltransferase as a first step express glycomimetics on their cell sur- guise. Several companies and academic toward glycorandomization. Nat. Struct. Biol. 8, faces, then using the antibiotic-coated bac- research teams are now developing anti- 545Ð551 (2001). 3. Weikert, S. et al. Engineering Chinese hamster teria as the drug. cancer vaccines that stimulate an immune ovary cells to maximize sialic acid content of recom- Many intracellular pathogens also response against the carbohydrate antigens binant glycoproteins. Nat. Biotechnol. 17, require sugars, and glycosylation is itself a found on tumor cells. Although these anti- 1116Ð1121 (2001). 4. Zitzmann, N. et al. Imino sugars inhibit the formation tempting target for novel antivirals. In one gens are normally tolerated by the immune and secretion of bovine viral diarrhea virus, a pes- promising approach being pursued by system, evidence is emerging that chemi- tivirus model of hepatitis C virus: implications for the development of broad spectrum anti-hepatitis virus Raymond Dwek and colleagues at Oxford cally modified glycans and adjuvants can agents. Proc. Natl. Acad. Sci. USA 12, University (UK), a small-molecule boost immunity, causing tumors to be held 11878Ð11882 (1999).

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